The present invention relates generally to light-emitting devices having organic electroluminescent material. In particular, the present invention relates to designs and methods for enhancing light extraction from such devices.
In the present disclosure, the terms “organic electroluminescent device” and “organic light emitting diode” are used interchangeably.
In the last decade, tremendous strides have been made in the science and technology of organic light emitting diodes (OLEDs). Most of this progress has been fueled by interest in developing flat panel displays, however, if the rate of progress can be sustained into the next decade, OLED technology has the potential to impact general lighting applications. In particular, a large area white-light emitting OLED could potentially provide a solid state diffuse light source competitive with conventional lighting technologies in performance and cost. Achieving this will require significant advances in efficiency, life at high brightness, and the illumination quality of OLED-generated light.
One of the limitations on organic light emitting diode (OLED) performance is the optical extraction efficiency, ηex which is the ratio of light generated within the device to light emitted into the ambient. Typical estimates for this efficiency factor in OLEDs range between 0.17-0.5.
Recently, a promising down-conversion method for achieving illumination-quality white light utilizing an underlying blue OLED was demonstrated. It was found that the presence of an inorganic phosphor layer actually enhanced the power efficiency of the underlying blue OLED and this was attributed to the presence of volume light scattering within the phosphor layer. In this paper, we develop an analytical model to understand the influence of volume scattering on OLED light output and use this to develop design guidelines to maximize device efficiency. In particular, we explore how the amount of light extracted from the OLED depends upon such factors as the particle size, loading fraction and optical loss in the system. In a companion paper, we describe detailed experimental measurements that prove the quantitative validity of this model.